BIREFRINGENCE AND STRUCTURAL PHASE TRANSITIONS IN ABCl3-TYPE CRYSTALS.

The refractive indices and birefringence of CsPbCl//3, TlMnCl//3, and CsSrCl//3 were determined a wide temperature range, which included phase transition points. Relatively high values of the photoelastic and acoustooptic characteristics were obtained: the th acoustooptic figure of merit M//2 was 12 multiplied by (times) 10** minus **1**8 cgs units for TlMnCl//3 and 98 multiplied by (times) 10** minus **1**8 cgs units for CsPbCl//3. A strong influence of partial Sr for Ca substitution on the properties and transition temperatures was observed. A phenomenological theory was used to analyze the relationship between the birefringence anomalies and the theromodynamic parameters of the transitions. A comparison was made with KMnF//3 and SrTiO//3.